The present invention generally relates to casting methods, and more particularly to a method of drying ceramic molds such as those used in an investment casting process.
Although a capital intensive and time-consuming process, investment casting employing the lost wax process permits high quality metal parts or components to be produced that include intricate details and configurations. Investment cast parts are used in the firearm, medical device, automotive, aerospace, manufacturing, power generation, oil and chemical, and other enumerable industries. Investment casting initially entails making meltable wax patterns of the metal parts desired to be manufactured by injecting wax in to a metal die. The individual wax patterns are removed and usually attached to a gating system or assembly called a sprue or stick that holds a plurality of patterns. The assembly is then dipped into a ceramic refractory slurry, which in some instances may contain a refractory flour, a colloidal silica binder, a latex polymer, and water which acts as a solvent (“water-based” binder/solvent system). In some applications, “alcohol-based” binder/solvent systems consisting of ethyl silicate and alcohol are used in lieu of colloidal silica and water. The assembly is then drained and dipped into dry refractory grains or “stucco.” The assembly is then dried to evaporate the solvent and gel the binder to produce a hardened ceramic “shell” layer. In order to produce a finished ceramic mold of sufficient thickness to ultimately withstand the thermal stresses induced by pouring hot molten metal into the mold to form the desired metal part, the dipping and drying process is repeated multiple times to gradually build up shell layer thickness to produce the final mold. After a ceramic mold of suitable shell thickness has been formed, the mold is dewaxed typically by using a high pressure steam (“autoclave”) or in a high temperature oven (“flash fire”). The mold is then heated or fired in an oven to cure or set the refractory material (“sintered”). This leaves a negative impression of the metal part to be cast in the mold. Finally, the preheated ceramic mold is filled with molten metal which solidifies into the shape of the desired parts. The expendable molds are then broken away to yield the cast metal parts.
A typical metal part formed by the foregoing investment casting process may in some instances require the formation of as many as seven shell layers or more of ceramic material to form a refractory mold of sufficient thickness. Because each shell layer must be thoroughly dried between each successive dipping into the ceramic slurry to at least gel the binder, the shell drying time between the multiple layers of ceramic shells significantly contributes to time and cost of producing the cast metal part.
The current industry standard used by foundries for drying the ceramic shell layers is air drying using low humidity, high velocity air. Using this conventional process, it may typically take up to three days or more from the formation of the initial prime ceramic coating or shell layer to the final dewaxing step. For a seven-layer shell, typical representative drying times may be about 2½ hours between layers 1 to 3, about 4 hours for layers 4 to 7, and 48 hours final drying. These drying times illustrate that the time required to dry each successive shell layer increases with the number of layers. Liquids in the ceramic slurry wick into previously dried coats of ceramic material. Therefore, with each successive shell layer built up in the ceramic mold, the required drying time increases because the liquid must travel farther from the previously dried shell layers to the surface of last dipped layer of the mold to be evaporated.
The required drying times are dependent upon factors such as temperature and relative humidity (moisture content) of the drying air, the air velocity, thickness of the ceramic shell layer (gradually increasing upon each successive slurry dipping and shell layer formation), and geometry of the metal part to be cast. For example, drying time increases with increasing relative humidity and vice versa. Lower airflow rates increase drying times. Relatively uniform drying of the ceramic shells is desired. More complex mold geometries and/or the presence of deep holes and slots, however, require longer drying times for the ceramic shells and adversely effect the ability to uniformly dry the shells.
The conventional air drying technique generally involves placing the molds in a temperature and humidity controlled environment, such as a room or enclosure that may incorporate drying fans for airflow control, supplementary heat sources, and humidity controls. The drying rooms are typically controlled to about 30-40% relative humidity to optimize drying times. Airflow requirements may vary from about 100 feet/minute for open and/or featureless molds to about 2,000 feet/minute for molds with deep holes or slots. It will be appreciated that these factors and poor airflow dynamics in drying rooms make it difficult to effectively control the drying rate and temperature of the ceramic shells, and to uniformly dry the molds.
Ideally, the ceramic shell drying process should also be controlled to minimize the temperature decrease of the wax pattern during drying. Stresses are created during drying because of differential thermal expansion between the wax and the ceramic shell material. For example, a temperature change from 70 to 100 degrees F. results in about 0.5% linear expansion for wax, but only less than 0.05% linear expansion for the ceramic. Accordingly, the more the wax cools during drying due to solvent evaporation, the larger the resulting stresses induced in the ceramic mold. High stresses can create detachment of the ceramic from the wax patterns “prime coat lift.” This produces castings that are scrap or require salvage to meet customer requirements. High stresses can also create cracks in the ceramic mold. These cracks, if not detected after dewaxing, can leak metal during pouring. Ideally, it is desirable to maintain as constant a wax temperature as possible and minimize temperature fluctuations to within a few degrees of ambient temperature.
Several alternative approaches have been identified to remedy the past problems associated with the conventional shell air drying method technique. These approaches, however, all have drawbacks. One such alternative technique is an elevated air temperature process in which the temperature and humidity of the air are closely controlled to maximize drying rate and minimize wax temperature change. Although this process can reduce shell layer drying time, it is cumbersome to implement. To set up a suitable program, a variety of wax patterns must be monitored to develop drying curves for optimizing the temperature and humidity process controls.
Another alternative drying approach to conventional air drying is the use of desiccants to improved the liberation of moisture from the ceramic shells. Such a system is shown in U.S. Pat. No. 3,755,915. However, desiccants which typically come in a granular form, are sometimes difficult to uniformly work into deeper apertures or recesses in the ceramic shell molds. In addition, such known systems failed to address the problems of mold heat gain that occurs during the moisture removal process with desiccants. Desiccants will actually generate heat due to the heat of adsorption principle involved as the desiccant adsorbs liquid from the ceramic molds. This may increase the temperature of the wax pattern to a point greater than desired to avoid damaging the molds.
Yet another alternative drying approach to conventional air drying is vacuum drying of ceramic shells. Although a vacuum conceptually would increase moisture removal from the ceramic shell and decrease the drying time, it concomitantly greatly increases evaporative cooling rates resulting in larger than desired temperature drops in the wax pattern. Accordingly, the vacuum process must be augmented by supplying external heat (e.g., microwave energy, radio frequency, cyclic vacuum with hot air purging, etc.) to counter-balance the ceramic shell heat loss and attempt to maintain a relative constant wax pattern temperature. Such processes are generally expensive, not readily adapted to commercial scale and production rates, requires additional equipment and capital, and increases energy consumption resulting in higher operating costs.
Accordingly, an improved method of drying ceramic shells in the casting process is desired.